Optical articles typically comprise a transparent optical substrate coated with an optional primer, an abrasion-resistant coating (or hard-coat) and possibly other layers such as an anti-reflection coating.
These optical articles are usually made of substantially insulating materials and tend to have their surface becoming easily charged with static electricity, particularly when cleaned under dry conditions by rubbing their surface with a wiping cloth, a piece of synthetic foam or of polyester. This phenomenon is called triboelectricity. Charges present on the surface thereof do create an electrostatic field able of attracting and retaining dust particles.
In order to counter this phenomenon, it is necessary to reduce the electrostatic field intensity, that is to say to reduce the number of static charges present on the article surface. This may be done by inserting into the stack of layers of the optical article a layer of a conducting material which dissipates the charges, also called an “antistatic coating”.
Such an antistatic coating may form the outer layer of the stack, or an intermediate layer thereof. For instance, it may be directly deposited onto the transparent optical substrate.
A typical antistatic material is TCO (transparent conductive oxide), which refers to an important class of photoelectric materials providing both electrical conductivity and optical transparency.
TCO materials can be divided into n-type (electron is charge carrier) and p-type (hole is charge carrier) materials. N-type TCO materials includes the Cd, In, Sn or Zn oxides or multiple complex oxides, which may be doped. Tin doped In2O3 (ITO) and antimony or fluorine doped SnO2 (respectively ATO and FTC), are among the most utilized TCO thin films in modern technology. In particular, ITO is used extensively for industrial applications. Recently, the scarcity and price of Indium needed for ITO has motivated industrial companies to find a substitute such as ATO, which has a lower cost than ITO. It has thus been suggested to use ATO for forming electrostatic coatings in optical articles (WO 2010/109154, WO 2010/015780). However, ATO has comparatively a lower conductivity and also a higher absorption in the visible light range than ITO. Consequently, ATO layers have limited transparency compared to ITO and a slightly blue coloration together with poorer conductivity. As another ITO potential substitute, FTO presents the advantage to be more conductive than ATO and have a higher transparency than ATO in the visible range (less absorption). Compared with other TCO materials, the FTO materials show also higher thermal stability, higher mechanical and chemical durability and lower toxicity.
TCO layers for industrial applications are commonly produced by PVD (Physical Vapor Deposition) (US 2012/295087). This method requires important investments in terms of production equipments (cost and space). Academic articles also report chemical deposition processes leading to ITO, ATO or FTO layers (J. Sol-Gel Sci. Technol., 2010, 53: 316-321; ACS Appl. Mater. Interfaces, 2012, 4(5): 2464-2473). However, these processes are starting from precursor solutions deposited on the substrate and drastic post-treatments like high temperature (around 600° C.) treatments are needed to obtain the crystalline TCO layer. These processes cannot be applied to plastic substrates, such as optical substrates, that are sensitive to temperature upper than 120° C. In view of the above, it would rather be desirable to be able to produce FTO layers at mild temperature, by wet deposition techniques, such as spin or dip-coating of FTO crystalline nanoparticles dispersed in a liquid. In particular, to be able to form very thin films (<100 nm) on the lenses or to introduce them in typical sol-gel formulations used for the formation of optical hard-coats, these particles should be able to be dispersed in alcoholic solvents.
The inventors have shown that such FTO layers could be prepared from a colloidal suspension of FTO obtained by a process comprising the steps of mixing tin and fluorine specific precursors and subjecting same to hydrothermal treatment. A similar process has already been described in CN101580270, wherein fluoride inorganic salts and tin nitrate, sulfate or chloride are used as fluorine and tin precursors, respectively. These precursors are mixed in water at a pH of 6-8 in the presence of H2O2 before subjecting them to hydrothermal treatment. The colloidal suspension obtained is then purified by ultrafiltration. However, colloids prepared according to CN101580270 are provided in an aqueous medium and cannot be stably dispersed in an alcoholic medium after solvent exchange. Moreover, they show quite high sheet resistance (higher than 100 Ω/square), which restricts their application. Finally, this process requires lengthy purification steps to eliminate the chloride ions used in large amounts, which negatively impacts the economics of this process.
Therefore, there remains the need to provide a simple process for preparing a stable colloidal suspension of fluorine-doped stannic oxide crystals in an alcoholic medium, which allows the formation of a thin, transparent and conductive film.
After conducting extensive research, the present inventors have demonstrated that this need could be satisfied by subjecting a mixture comprising a specific type of organic fluorine precursor and stannous oxalate to hydrothermal treatment.